The continuous simulated moving bed process has been disclosed already in U.S. Pat. No. 2,985,589 (Broughton et al.). In accordance with this process the mixture to be fractionated is introduced into one partial packed bed and eluant is introduced into another partial packed bed, and two product fractions are withdrawn substantially simultaneously. There are at least four partial packed beds, forming a single loop with continuous circulation of a dry substance profile, and the feed and product withdrawal points are shifted continuously stepwise in the downstream direction in the loop of packing material beds essentially by the speed of the circulation of the dry substance profile in the loop.
For simulated moving bed chromatographic separation processes two or more loop and two or more profile modes have been developed in order to better utilise the chromatographic separation resin bed to achieve increased separation capacity, increased yields and fraction purities and fraction dry substance concentrations.
A sequential simulated moving bed process applied to the recovery of betaine and sucrose from beet molasses is described in the Applicants' Finnish Patent 86 416. In this method, one complete or essentially complete dry solids profile is circulated in a partial packing material loop. Also the Applicants' publications U.S. Pat. No. 6,093,326 and U.S. Pat. No. 5,637,225 relate to a simulated moving bed method, the first applied to the fractionation of molasses and the latter to the fractionation of sulphite cooking liquor. As is described in these patents, the simulated moving bed method may include multiple loops; and dry solids profile(s) is circulated in each loop.
Finnish Patent 86 416 referred to above discloses a method for recovering betaine and sucrose from beet molasses employing a simulated moving bed process. The chromatographic system comprises at least 3 chromatographic partial packed beds in series. In the method, betaine and sucrose are separated during the same sequence comprising a molasses feeding phase wherein the molasses feedstock is supplied to one of said partial packed beds and eluant water is supplied substantially simultaneously to another of said partial packed beds, an eluant feeding phase, and a circulating phase. These phases are repeated either once or several times during the sequence.
In the method disclosed in the above-stated U.S. Pat. No. 6,093,326, the liquid flow is effected in a system comprising at least two partial packed beds, and the product or products are recovered during a multistep sequence. A sequence comprises feeding, eluting and circulation phases. During the circulation phase, the liquid present in the partial packed beds with its dry solids profile is circulated in two or more loops comprising one, two or more partial packed beds. A loop may be closed or “open”, in other words, when liquid is circulated in one loop, eluant can be introduced into the other loop and a product fraction can be withdrawn therefrom. During the feed and elution, the flow through the packing material beds may take place between successive loops, loops, wherein the flows carry material from one loop to another. During the circulating phase, column or columns form closed loop and are separated from the other loops. Essentially complete dry solids profile is circulated in each loop.
Publication WO 97/45185 discloses a method for fractionating a solution into two or more fractions by a chromatographic simulated moving bed process wherein the separation system comprises at least two separation profiles in the same loop. The method can be used for fractionating a sulphite cooking liquor to give a fraction rich in monosaccharides and/or a fraction rich in lignosulphonates. Furthermore, molasses or vinasse can be fractionated in that way to obtain fractions rich in sugar, such as sucrose, and/or betaine. In the method of the above mentioned publication, there are at least two separation profiles in the same loop. The minimum required bed length for the method is at least the length of two separation profiles without excess overlapping.
One of the problems associated with the above arrangements is that in order to get good separation result, high yields, purities and product fraction dry substance concentration, long chromatographic separation beds have to be used.
Another problem with the earlier arrangements is also that the dry substance profile(s) in the chromatographic separation beds do not necessarily fill the whole bed length or they may overfill the bed. As a rule, the dry substance profile should be formed so that high product fraction yields, purities, concentrations and high separation capacities are achieved. This means that chromatographic column load has to be high, e.g. the feed volume has to be large, in order to achieve high product fraction dry substance concentration and high separation capacities. High dry substance concentration means that the concentration is at least close to the partial concentration of the component(s) in the feed.
On the other hand, chromatographic separation bed(s) height has to be long to contain high enough number of theoretical plates in order to achieve high product purities and product yields.L=N·HETP  (1)N=16·(Ve/W)2  (2)
wherein L=length of the resin bed                N=number of the theoretical plates        HETP=height equivalent of the theoretical plate        Ve=elution volume        W=peak width.        
In one situation the separation profile is narrow and chromatographic separation bed is long which may lead to a situation where part of the chromatographic separation column is not loaded with the separation profile causing poor utilisation of the bed.
Alternatively, if in another situation the dry substance profile is long and chromatographic separation bed needed for good separation is short leading to the overlapping of the dry substance profile from both ends of the profile occurs causing low purities and low yields.
In the above mentioned situations more advanced methods are required and the present invention provides a solution to the problems associated with both situations.